20 • JATO 3.3
Air
Fuel
See page 26 for more
information on how air
density affects engine tuning.
See
Tuning Your TRX 3.3
Racing Engine
on page 25
for complete information on
adjusting the air/fuel mixture
and idle speed.
THE CARBURETOR
Understanding the Carburetor adjustments
The carburetor performs several functions. It controls the engine’s
speed by restricting the intake of air and fuel into the engine. It
atomizes the fuel (suspends the fuel droplets in the air) and also
controls the air/fuel ratio of the mixture entering the engine (how
much air for a given amount of fuel).
To help provide a better
understanding of engine
tuning and why it’s
necessary, the following is a
brief explanation of the air/
fuel combustion process that
takes place inside the engine.
In order to create the cylinder pressure that
results in power, the engine burns the air/
fuel mixture. Both air and fuel, in correct amounts, are needed for
proper combustion. It is the carburetor’s job to mix the air and fuel
together (atomize the fuel), in the correct proportion for the best
possible combustion. This is the ideal air/
fuel ratio. The ideal air/fuel ratio required
for the engine remains roughly constant.
Due to variations in atmospheric conditions
(temperature, humidity, altitude etc.) fuel
flow adjustment valves (called fuel mixture
needles) are required to meter the fuel and
maintain the ideal air/fuel ratio in these
ever-changing conditions. For example,
colder air is more dense (more air molecules)
for a given volume of air and therefore
requires more fuel (more fuel molecules) to
maintain the correct air/fuel ratio. Warmer
air is less dense (fewer air molecules) and
therefore needs less fuel to maintain the
correct air/fuel ratio. The tuning needles
are there to adjust how much fuel is made
available for the carburetor to mix with the
available air (atomization).
THE FUEL MIXTURE NEEDLES
The amount of fuel metered and atomized by the carburetor is
controlled by the two mixture needles, the high-speed needle and
the low-speed needle. The low-speed needle is used to meter the
fuel used by the engine at idle and low rpm (part-throttle) engine
speed. The high-speed needle is used to meter the fuel when the
throttle is open from part throttle to wide-open throttle (WOT). Two
needles on the TRX 3.3 Racing Engine provide precise control of the
air/fuel ratio across the engine’s entire rpm range.
The maximum possible fuel flow is always controlled by the high-
speed needle. It works like the main water valve on a garden hose.
Turn it clockwise to close the valve, counter-clockwise to open it.
When the throttle is at idle or partially open, the low-speed needle
meters the fuel flow at the outlet (needle seat) where the fuel enters
the carburetor venturi. This second valve acts like the spray nozzle
at the end of the garden hose in our example. When you accelerate
from idle, the throttle opens and the low-speed needle is pulled
away from the needle seat. This allows more fuel to flow with the
increased air flow. As the throttle is increased, the low-speed needle
is pulled completely away from the needle seat leaving it fully open.
At that point, fuel metering is entirely controlled by the high-speed
needle. Again, using our water hose example, when the spray nozzle
at the end of our garden hose is fully open, then the main water
valve can be used to adjust how fast the water flows.
The engine’s performance is directly linked to the fuel mixture.
Richening the fuel mixture increases the amount of fuel in the
air/fuel mixture ratio and leaning the fuel mixture decreases the
amount of fuel in the air/fuel mixture ratio.
Water Valve
(Controls Overall Flow)
High Speed Needle
Water Nozzle
(Controls Fine Flow)
Low Speed Needle
Richen
Cold Air (More Dense)
(Warm Air) Less Dense
Start
1/8 Turn
45°
Full Turn
360°
1/2 Turn
180°
1/4 Turn
90°
3/4 Turn
270°
A “turn” refers to
tightening (“turning in”) or
loosening (“turning out”)
mixture needles. A “full
turn” refers to turning the
needle 360°, so a “1/2 turn”
would be 180°, a “1/4 turn”
would be 90°, and so on.
THE TRX 3.3 RACING ENGINE
